The role of endogenous opiates in regulating anterior pituitary hormone secretion will be examined. Of primary interest is the phenomenon of tolerance following acute and chronic opiate exposure. The mechanisms of this process, the site and the involvement of end target hormone feedback will be investigated. Changes in hormone secretion will be correlated with alterations in analgesia and body temperature. An assessment of the physiological consequences of opiate exposure on the endocrine responsiveness of the organism will be carried out. Of additional interest is the type of ligand participating in hormone secretion. A characterization of the receptor class and endogenous opiate (endorphine or enkephalin) involved in regulation of each pituitary hormone is anticipated.